[zkool448]

thanks all, want to crosspost it here also. -zk

___

Hi everyone, wow... truly overwhelmed by your responses. I read each of your post and really means a lot to me guys. Thank you.

Special thanks to Alex Cavalli for designing and producing such a great sounding amp.

cheers -zk

In response to the questions/comments, the mounting bracket for the headscrews aren't installed as I wanted to monitor the cooling first. I'm happy to report that the tapped L-brackets will be added permanently -- the amp has been burning in for 10+ hrs without any heat issues! The top/bottom lids which appear 'metal' is really mdf wood machined using a table router.

Casework felt like it took forever, won't be selling them but more than happy to help if you're looking for sme wood design ideas (ex. #1, #2), cutting tips, shop tools, etc.

Smeggy, I know nothing about hooking up a potted torroid (or any electronics for that matter), that option would've been awesome if I knew how to. Your woodwork finishing tips is what inspired the finish

Adamus, the socket savers works well indeed. Really helps in the cooling department since the tubes are fully (or almost) exposed.

Thanks again to all DIY'er out there for their help, inspiration, tips ,and amp designs. It's now time for a nice break to enjoy the amp!

 

[Alcaudon]

Quote:

Originally Posted by runeight /img/forum/go_quote.gif You can do this but remember that the LEDs come from the heater supply and are negative with respect to ground. You can pull from the +15V supply but the reg will burn 50% more power when you do. You might want a bigger heatsink.

Many thanks for the answer runeight!!!!

 

[holland]

Quote:

Originally Posted by wolf18t /img/forum/go_quote.gif Keep us informed if this higher tail current tweak make any positive difference

I put a 56 ohm resistor in parallel, just tacked it on top. I was too lazy to remove the old one.

Anyhow, I was running at the max of the 200 ohm resistor for a while, and can't say I noticed a difference. However, I think I notice some differences in tone now. I'm running 10mA through the tail (5mA per triode). Things seem to be a bit more "extended" up top. I'm using a 6CG7 at the moment.

Then again, it could all be placebo.

You know how audio memory is.

 

[wolf18t]

Quote:

Originally Posted by holland /img/forum/go_quote.gif I put a 56 ohm resistor in parallel, just tacked it on top. I was too lazy to remove the old one. Anyhow, I was running at the max of the 200 ohm resistor for a while, and can't say I noticed a difference. However, I think I notice some differences in tone now. I'm running 10mA through the tail (5mA per triode). Things seem to be a bit more "extended" up top. I'm using a 6CG7 at the moment. Then again, it could all be placebo. You know how audio memory is.

Placebo effect or not, you heard an improvement and that's what really count.

Seriously, thanks for the follow up Holland.

 

[zkool448]

SOHA II update:

I didn't realize my SOHA II had a mild hum/buzz noise. Been using a DAC, sometimes the iPod ever since the build and didn't hear any hum/noise/hiss at all until last week when I tried my desktop sound card as source. No problems after I installed a wirewound 10R 5watts parralel with .1uF film cap between SG and IEC ground. Got rid of the hum completely.., I'm again a very happy camper.

while I was tweaking around, thought I'd do some tail current adjustments with a pair of zeners. Swapped R4's with 18V 0.5W zeners but once I powered up there was a very audible hiss (regardless of volume level), so I just went back to the resistors as only way to rid of it.

Lastly, I swapped the stock BOM caps (C5L/C5R & C6P/C7P/C12P) to Nichicons/Panasonic FMs -- ever since then I noticed the ε12 relay no longer triggers (?) I can crank the volume up to levels I couldn't set to before (read: unbearably loud). Is there a way fo me to test this to check or make sure it still acting as a protection circuit?

 

[adamus]

i take it is still delaying the output?

 

[zkool448]

Yes on power on, I hear a delayed click after ~7-8 seconds.

 

[Ferrari]

Quote:

Originally Posted by zkool448 /img/forum/go_quote.gif SOHA II update: Lastly, I swapped the stock BOM caps (C5L/C5R & C6P/C7P/C12P) to Nichicons/Panasonic FMs -- ever since then I noticed the ε12 relay no longer triggers (?) I can crank the volume up to levels I couldn't set to before (read: unbearably loud). Is there a way fo me to test this to check or make sure it still acting as a protection circuit?

The caps you have replaced (C5L/C5R & C6P/C7P/C12P) are all PS filter caps. As long as the voltage rating is adequate and they fit the board then it’s OK. These caps have no impact on the behavior of the ε12 circuit.

To verify the other function of ε12 circuit -as a DC offset protection circuit- you can use the trimpot P1P in the B+ section to vary the B+ voltage. In/decreasing the B+ voltage quickly will result in transient which will trigger the ε12 circuit to shut down the output.

 

[holland]

Ferrari, IIRC you're running the B+ @ 125V. What did you do to the CW multiplier to get that? Just add another rung to the ladder and swap the odd and even sides so you can end on a cap?

I have the prototype board, still. My plan, and final mod for the SOHA II before putting it away, is to do another HV perfboard that removes the TL783, and add back the 3rd cap multiplier in the original design to run B+ @ ~140V. Cap upgrades to 160V every where on the B+ string.. I never had the problem that required the TL783, but since the HV is external, it's easy to tweak and regulate to 125V if necessary....which is what you are doing, I believe.

Thanks!

 

[wiatrob]

Quote:

Originally Posted by zkool448 /img/forum/go_quote.gif SOHA II update: while I was tweaking around, thought I'd do some tail current adjustments with a pair of zeners. Swapped R4's with 18V 0.5W zeners but once I powered up there was a very audible hiss (regardless of volume level), so I just went back to the resistors as only way to rid of it.

That's the Zener noise. dBel even rigged up some low noise zeners and we sent them round during the proto phase, but never could quite get rid of the noise. The level of noise does appear to differ depending on setup, on my first proto I could barely hear it with the volume all the way up.

 

[holland]

FWIW, I found the low noise zener to be noiser than the 18V zener. Go figure. Either way, the zener seems to cause hiss no matter what, as wiatrob indicated. Since I've dialed mine up to 10mA, I just need to drop in some 3.6K resistors and that'll be set.

I have zener diodes in mine, at the moment. I'm using Denon D2000 headphones and the hiss is audible. It's not horribly loud, but it's there. I can't really hear it with music, but between tracks, definitely. I agree that it can be annoying.

 

[Ferrari]

Quote:

Originally Posted by holland /img/forum/go_quote.gif Ferrari, IIRC you're running the B+ @ 125V. What did you do to the CW multiplier to get that? Just add another rung to the ladder and swap the odd and even sides so you can end on a cap? I have the prototype board, still. My plan, and final mod for the SOHA II before putting it away, is to do another HV perfboard that removes the TL783, and add back the 3rd cap multiplier in the original design to run B+ @ ~140V. Cap upgrades to 160V every where on the B+ string.. I never had the problem that required the TL783, but since the HV is external, it's easy to tweak and regulate to 125V if necessary....which is what you are doing, I believe. Thanks!

Actually I’m running the B+ voltage of my amp at 120V DC to stay within the limit of the TI TL783 regulator (125V).
The higher voltage can be achieved by an additional stack (C-D) on the existing CW voltage multiplier, or by using a transformer with higher secondary voltage while let the CW voltage multiplier unchanged. (Transformer with 2x18V secondary resulted in ~138V unregulated B+ on my amp).

I have experimented both ways, but finally go for the higher secondary voltage transformer solution.
On the symmetrical low voltage section, MC7818/7918 regulators were used instead of 7815/7915.
For the OpAmps PS rails, a diode was added on each rails to keep the voltage a bit lower (- 0.7V) than ±18VDC (a couple of traces cuts needed).
The higher voltage has of course impact on the heat dissipation on the output stage, so don’t forget to pay attention to heatsinking.

 

[zkool448]

Quote:

Originally Posted by Ferrari /img/forum/go_quote.gif To verify the other function of ε12 circuit -as a DC offset protection circuit- you can use the trimpot P1P in the B+ section to vary the B+ voltage. In/decreasing the B+ voltage quickly will result in transient which will trigger the ε12 circuit to shut down the output.

Wonderful Ferrari, e12 is still working indeed -- thanks

Holland, yes the hiss is definitely a show stopper for me.

 

[Arkku]

As for the zeners and noise, here's a bit more on my experiments with them:

When I first built the amp, I tried both the resistors and Fairchild 1N5248B zeners. I couldn't detect any difference between them, so I left the zeners in. Recently I tried tampering with the tail current (with zeners installed), and I noticed that even a 50% increase caused very annoying and very audible hiss. Take it back down to the default and it's silent again. (I also noticed that I can't set the tail current very high; at twice the default e12 trips at very low volumes while normally I can pretty much turn the knob all the way to unbearably loud, no problem.)

Now I'm running with the zeners at default settings again, and no noise. With headphones, that is; I also use this as a pre-amp, and in that use there's a slight hum audible in a quiet room with no signal. It's at a constant volume regardless of volume knob position or input, so it could be dealt with by simply padding the output enough to mute the noise and then set the volume a bit higher (but I haven't gotten around to it). The reason I mention this noise is to note that the zener vs resistor has no effect on it.

So, it is possible to use zeners in this amp with no noise. (Just don't know if you can hear any difference. =)

 

[Ferrari]

Finally having the time to test my über SOHA II. As expected, all circuits work correctly at first power up.